13-roman-to-integer.js

/**
 * @param {string} s
 * @return {number}
 */
const romanToInt = function(s) {
  let res = 0
  for (let i = s.length - 1; i >= 0; i--) {
    let c = s.charAt(i)
    switch (c) {
      case 'I':
        res += res >= 5 ? -1 : 1
        break
      case 'V':
        res += 5
        break
      case 'X':
        res += 10 * (res >= 50 ? -1 : 1)
        break
      case 'L':
        res += 50
        break
      case 'C':
        res += 100 * (res >= 500 ? -1 : 1)
        break
      case 'D':
        res += 500
        break
      case 'M':
        res += 1000
        break
    }
  }
  return res
}


// another

/**
 * @param {string} s
 * @return {number}
 */
const romanToInt = function(s) {
  const map = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 }
  let result = 0
  let index = s.length - 1
  let preInt = 0
  while (index >= 0) {
    let ch = s[index]
    let curInt = map[ch]
    if (curInt >= preInt) result += curInt
    else result -= curInt
    preInt = curInt
    index--
  }
  return result
}